1. Field of the Invention
This invention relates to non-destructive testing methods and apparatus in general, and in particular, to the use of optical fibers as an indicator of damage to filament wound composite pressure vessels.
2. Related Art
A number of important fiber reinforced composite structures are currently being manufactured using filament winding. They are made in a broad range of sizes, and are categorized primarily by their application. Aerospace applications include vessels for high-pressure gas containment, liquid propellant tanks, and solid rocket motors. Such vessels are also used extensively in non-Aerospace applications, but are normally designed to be much more robust, as weight control is typically not as critical. Currently, solid rocket motor cases of up to 5 feet in diameter are being made, and even larger sizes are possible. They typically have an elastomeric liner/insulator on the inside of the case wall.
Another class of composite pressure vessels are made for containment of gasses at high pressures, and are usually relatively small, typically less than two feet in diameter. Those with thin metal liners are defined by the American Institute of Aeronautics and Astronautics (AIAA) in specification S0-81 as “Composite Overwrapped Pressure Vessels,” or “COPVs”. There are also high pressure gas containment composite pressure vessels with non-metallic liners that have been used for commercial applications. Very large liquid-containing propellant tanks have been made, and these usually have a flexible liner on the composite wall for fuel containment. Typically, an epoxy-wetted fiber is wound in specified hoop and helical patterns on either a metallic liner, in the case of COPVs, or on a removable mandrel, and either with or without a liner at the winding stage. Features common to all composite pressure vessels used in aerospace applications are their relatively thin structural walls and thin metallic or elastomeric liners, which are used to minimize weight, and which render them relatively susceptible to shock and impact damage.
A preferred fiber for filament wound structures is carbon (graphite) fiber, which is exceptionally strong, but local deflections are limited before breakage occurs. Surface impact events experienced by these structures can cause significant local deflections resulting in broken fibers. The deflections can be elastic in nature, with no visual indication of subsurface fiber breakage or surface damage evident. The reduction in strength of the composite structure resulting from broken fibers can have implications that range from the need for relatively low-cost repairs, to a catastrophic failure of the vessel that can lead to calamitous failure of a flight vehicle. Unfortunately, not all impact events that occur subsequent to production inspection and that result in broken fibers are visually observed and reported through appropriate channels so that a detailed inspection is initiated to evaluate vessel integrity.
The susceptibility to impact damage of composite pressure vessels can be mitigated somewhat through design by providing for adequate strength after visible impact damage has occurred and been confirmed by visual inspection. However, for some high performance composite structures that are weight critical, such measures are not feasible, and multiple inspections are therefore necessary to ensure structural integrity. Accordingly, some space vehicle programs may expend very large sums of money each year to re-inspect solid rocket motor cases for impact damage using traditional non-destructive inspection methods prior to launch. Because of these high costs, new apparatus and methods for detecting impact or handling damage in composite structures are needed.
Some efforts have been made in this area to decrease the time and cost of traditional non-destructive inspections, including embedding sensing devices into woven fiber cloth, or co-curing them between laminate plies. (See, e.g., U.S. Pat. No. 5,814,729 to Wu, et al.; U.S. Pat. No. 5,245,180 to Sirkis; Claus, et al., “Nondestructive Evaluation of Composite Materials by Pulsed Time Domain Methods in Imbedded Optical Fibers,” Review of Progress in Quantitative Nondestructive Evaluation, Vol. 5B, Thompson and Chimenti, Eds., Plenum Press, 1986; Maslouhi, et al., “Use of Embedded Optical Fiber Sensors for Acoustic Emission Detection Within Composite Materials,” 36th International SAMPE Symposium, Apr. 15–18, 1991.) These references describe so-called “smart structures” that are equipped with sensing devices that indicate when a ply has been damaged. The drawback of these structures lies in the embedding within them of fiber optic sensors, which are typically larger than the adjacent structural ply fibers, and which therefore can adversely affect the strength of the lay-up or sustain embedded fiber damage during the curing process.
A need therefore exists for a low cost yet reliable method and apparatus for detecting shock, impact, handling and transportation damage to composite pressurized structures.